Optical Coherence Tomography (OCT) is an emerging technique for sub-surface imaging with medical, biological and other applications. OCT is the optical analog of ultrasound but takes advantage of the shorter wavelengths of light to achieve higher resolution images. Potentially the wavelength ranges of interest lie within the visible to near infrared regions of the spectrum from 400-2000 nm. Currently, there are four wavelength ranges of interest in OCT centered around 850 nm, 1050 nm, 1300 nm and 1550 nm. OCT systems comprise (1) a broadband light source, (2) an arrangement of optical components for directing the emitted radiation to a sample and to a reference mirror and (3) an optical arrangement for measurement of interference of light reflected from the sample and light reflected from the reference mirror. For state-of-the-art Time Domain OCT (TD-OCT) systems, the broadband source is a superluminescent light emitting diode (SLED) which emits light in a broad spectrum of wavelengths (40-200 nm bandwidth), and the sub-surface imaging depth is controlled by scanning the position of the reference mirror. Constructive interference only occurs when the path-lengths between the reference mirror and sample reflectors are equal within the coherence length of the light source. Fourier or Frequency domain OCT (FD-OCT) makes use of spectral frequency information, for example by decoding the interference using a dispersive element and a CCD detector array or Spectrometer (Spectral Domain OCT or SD-OCT). Another FD-OCT technique, requiring a simpler detection system, uses spectral encoding in time by spectrally scanning a narrow bandwidth light source (Swept Source OCT or SS-OCT).
According to the state of the art, in SS-OCT, a tunable laser is used as the light source. SS-OCT has been demonstrated to have major signal to noise advantages over TD-OCT (see Choma et al, Optics Express, vol. 11, 2003 pp. 2183-2189). To realize its advantages over TD-OCT, swept source OCT systems require tunable light sources that can be swept across a wavelength range at high frequencies of 20-400 kHz. Since the imaging depth required for the interferometers of these systems is related to the coherence length of the light source, this also sets the requirements for the linewidth and line spacing of the light source.
Various methods have been employed to achieve swept sources for SS-OCT. Among these are cascaded Distributed Feedback (DFB) lasers (US 2008/0037608), multi-wavelength lasers (US 2007/0002327), diffraction gratings and grating pairs (US 2008/0002209, U.S. Pat. No. 7,006,231), fiber laser ring cavities (US 2006/0193352) external cavity lasers employing Fabry-Perot filter tuning elements in both ring and linear configurations (U.S. Pat. No. 7,242,509, US 2006/0215713). Of these methods, the latter MEMS Fabry-Perot filter approaches are most applicable to integration within compact optical modules. In this case, the MEMS device performs both wavelength selection and wavelength sweep functions and needs to be manufactured to tight mechanical tolerances. The Fabry-Perot MEMS device is essentially a linear transducer that has speed limitations and is susceptible to variations in reflectivity, parallelism, flatness and filter cavity thickness over the tuning bandwidth. Variation in the latter parameter results in variation in bandwidth, coherence and imaging depth during a wavelength sweep of an SS-OCT system.
Another method (US 2007/0183643) employs a MEMS tunable mirror integrated with a Vertical Cavity Surface Emitting Laser (VCSEL). Integration of these elements, however, is difficult to manufacture and has yet to be realized.
Simpler and more versatile designs are based on external cavity lasers where the filtering and wavelength sweep functions are performed by a grating and a rotating or scanning mirror (U.S. Pat. Nos. 6,111,645, 5,907,423, US 2007/0276269, US 2007/0239035, US 2007/0064239, US 2008/0043244, US 2004/0213306). These designs are versatile, since they can relatively easily be adapted for other wavelength ranges. Swept-source OCT applications require the swept source to have a coherence length that is above a certain minimum value and below a certain maximum value. The minimum value is related to the minimum imaging range of the application (imaging range is half the coherence length), for example 6 mm for retina imaging or up to 25 mm for imaging of the whole eye from the retina to the cornea. However, it would not be good to take a swept source that has a too long coherence length because then any kind of reflective object within the imaging range will cause unwanted interference signals. Therefore, multimode laser operation is required in order for the spectral bandwidth of the external cavity laser to have a certain minimum value. Single mode operation would not guarantee that the coherence length does not exceed a certain maximum acceptable value. A consequence of this minimum spectral bandwidth is a relatively long cavity length. In general, this lies in a range 3-30 cm, depending upon the wavelength range and the number of modes in the cavity. This cavity length requirement restricts the miniaturization of the swept source. Ideally, the swept source should easily be qualified and integrated into compact SS-OCT systems. Also, a too long cavity length makes the wavelength tuning slow, limiting the sweep rate.